GROUND WATER ATLAS of the UNITED STATES
Arkansas, Louisiana, Mississippi
HA 730-F

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REGIONAL SUMMARY

INTRODUCTION

The States of Arkansas, Louisiana, and Mississippi, which are
located adjacent to each other and north of the Gulf of Mexico,
compose Segment 5 of this Atlas. The three-State area encompasses
an area of nearly 149,000 square miles. These States are drained
by numerous rivers and streams, such as the Atchafalaya, the Teche,
the Vermilion, the Calcasieu, the Mermentau, the Sabine, the Tombigbee,
the Pascagoula, the Wolf, and the Pearl Rivers, that drain directly
to the Gulf of Mexico. The Yazoo, the Big Black, the Arkansas,
the St. Francis, the Red, and the White Rivers are tributaries
of the Mississippi River, which is the largest of the rivers that
drain the three States. Although surface water is the largest
source of freshwater to public supply, domestic and commercial,
industrial, mining, thermoelectric power and agricultural users,
ground water also is important and accounts for 38 percent of
total water use in Arkansas, Louisiana, and Mississippi.

Precipitation is the ultimate source of water that recharges
the ma-jor aquifers in Segment 5. Average annual rainfall (1951-80)
amounts range from about 40 to about 68 inches (fig.
1). Temporal (seasonal) and spatial variations in precipitation
are evident in the three-State area.

Average annual rainfall is greatest (60 inches per year or
more) in southern Louisiana and southern Mississippi and diminishes
in Arkansas and in northwestern Louisiana. Precipitation is greatest
during January and May in Arkansas. May to September represent
the wettest months in southeastern Louisiana and southern Mississippi.
March and April are the wettest months in northern Mississippi.
Average annual (1951-80) runoff ranges from less than 12 inches
in western Louisiana and northwestern Arkansas to more than 20
inches in southern and northern Mississippi and in central and
western Arkansas (fig. 2). Comparison
of precipitation and runoff maps shows that less than one-half
of the annual precipitation leaves the area as stream runoff.
Much of the water that does not exit Segment 5 as runoff is returned
to the atmosphere by evapotranspiration, which is the combination
of transpiration by vegetation and evaporation from marshes, swamps,
lakes and streams. A small amount of water recharges aquifers
that are either exposed or buried to shallow depths, and an even
smaller amount percolates downward and enters the deep flow system.

PHYSIOGRAPHY

Segment 5 contains parts of four physiographic provinces-the
Coastal Plain, the Ouachita, the Ozark Plateaus, and the Interior
Low Plateaus (fig. 3). Uplands of the
Ouachita and the Ozark Plateaus Provinces occupy the northwestern
one-half of Arkansas. The Fall Line, a physiographic boundary
that marks the inner margin of the Coastal Plain, separates the
two provinces from the lowlands of the Coastal Plain Province.
The Interior Low Plateaus Province is present in only a small
part of the northeasternmost Mississippi and is discussed in Segment
6. The most extensively utilized aquifer systems of Segment 5
underlie the Coastal Plain Province.

The Mississippi Alluvial Plain Section separates the eastern
and western sections of the Gulf Coastal Plain Province. The Mississippi
Alluvial Plain consists of a low flood plain and delta system
that were formed by the Mississippi River. Crowley's Ridge, Arkansas,
is the most prominent topographic feature within the Mississippi
Alluvial Plain and is, in part, a north-south outlier of older,
underlying Coastal Plain rocks. The southern portion of the ridge
is covered with loess that is thought to have been deposited at
the same time as the river terraces. The ridge cuts the northern
part of the alluvial plain in half and is thought by some workers
to have formed when the Mississippi and the Ohio Rivers flowed
on opposite sides of the ridge. The Mississippi River captured
the Ohio River along an upstream reach during late Pleistocene
time, which reduced the river complex to one principal channel.
Recent workers suggest Crowley's Ridge may be the result of Holocene
fault movement. Although the Mississippi River is the principal
river of the Mississippi Alluvial Plain Section, the Tensas, the
Sunflower, and the Yazoo Rivers are among several other streams
whose drainage basins are entirely or mostly contained within
the alluvial plain. The distributary part of the Mississippi River
system is in southern Louisiana. Deposition of sediments along
and between distributaries has created a large delta, whose shape
is best described as a bird-foot delta (fig.
4). The delta extends east-southeast and has built outward
atop thick marine clay beds into the Gulf of Mexico. Thick sandy
distributary channels are separated by interdistributary deposits
of muds, thin muddy sands, and abundant organic deposits. The
weight of the advancing delta front sand compacts thick underlying
clays and forms depressions in which prograding channel sand or
bar-finger sand facies are protected from erosion. As progradation
continues, distributary channels extended further seaward, and
the delta enlarges.

The altitude of most of the inland part of the Coastal Plain
Province ranges from 300 to 600 feet above sea level in Mississippi
and from 200 to 400 feet above sea level in Arkansas and Louisiana
(fig. 5). A 10- to 150-mile wide coastal
zone immediately adjacent to the Gulf of Mexico lies no more than
50 feet above sea level. The eastern part of the Gulf Coastal
Plain Province (fig. 3) is characterized
by a coastal plain of low hills, low cuesta ridges, and gentle
lowlands. Fine-grained strata of clay, chalk, and mudstone underlie
the low-lying areas; coarse sand and gravel underlie low ridges
and hills. The western part of the province is a southward-facing
plain of low, rolling, slightly hilly terrain that becomes a flat
plain to the south. A broad marshy zone is near the coast.

The Ouachita Province in Arkansas is north of the West Gulf
Coastal Plain Section and can be separated into the Ouachita Mountains
Section to the south and Arkansas Valley Section to the north
(fig. 3). The Ouachita Mountains Section
is distinguished by valley and ridge topography. The ridges form
straight to zigzag patterns and increase in height westward. Some
ridges rise to more than 2,000 feet above sea level. North of
the Ouachita Mountains Section, the Arkansas Valley Section forms
a low-lying plain with low ridges oriented east to west. Although
much of the Arkansas Valley Section generally is only 300 to 600
feet above sea level (fig. 5), the
altitudes of several ridges range from 1,000 to more than 2,000
feet.

The Ozark Plateaus Province is north of the Ouachita Province
and can be separated into the Boston Mountains Section to the
south and the Springfield and the Salem Plateaus Sections to the
north. The Springfield Plateau lies west and south of the Salem
Plateau. The 200-mile long by 35-mile wide Boston Mountains Section
is a deeply dissected plateau region that generally ranges from
more than 1,900 to more than 2,500 feet above sea level and is
characterized by flat-crested ridges that rise from 300 to more
than 1,000 feet above V-shaped valleys. The surface of the Western
Springfield Plateau, which is the intermediate level plateau,
varies from gently rolling prairies to dissected terrain that
ranges from 1,000 to 2,000 feet above sea level along its northern
and southern margins. Topographic relief within this plateau area
ranges from less than 100 feet in the prairie areas to more than
400 feet where streams have incised a north-facing escarpment
that borders the Salem Plateau. In some areas of the Springfield
Plateau, straight solution valleys intersect one another at 90-degree
angles. The Salem Plateau, which is located east and north of
the Springfield Plateau, lies at altitudes of 1,000 feet or less
above sea level, but its land surface forms an irregular topography
and is cut deeply by streams. However, topographic relief between
hill crests and valley bottoms usually does not exceed 100 feet.

MAJOR AQUIFERS

Major aquifers in Segment 5 are highly varied in composition,
consolidation, and hydraulic character. The majority of Segment
5 aquifers consist of unconsolidated to poorly consolidated Coastal
Plain strata of gravel, sand, clay, and minor limestone of Cretaceous
to Holocene age. Other Segment 5 aquifers consist of indurated
limestone, dolomite, shale, sandstone, chert, and novaculite of
Paleozoic age that are either flatlying or gently to highly folded
and contorted and that may be faulted and fractured. These aquifers
are combined into eight aquifer systems, all of which extend beyond
the Segment 5 study area (fig. 6).
Only small parts of the Texas coastal uplands and the Floridan
aquifer systems are in Segment 5; these aquifer systems are discussed
in Chapters E and G of this Atlas, respectively.

An aquifer system consists of two or more aquifers that are
hydraulically connected. The aquifers may be separated, in places,
by confining units, but there is regional hydraulic continuity
within the system-the flow systems of the aquifers function similarly,
and a change in conditions within one aquifer commonly affects
the other aquifer(s). Likewise, confining units that may contain
local aquifers, but which function together to retard the vertical
movement of water, are called confining systems. The outcrop extent
of the principal aquifers, aquifer systems, and a confining system
in the Segment 5 study area is shown in figure
7.

The surficial aquifer system consists of alluvial aquifers
and includes one major and three minor aquifers (fig.
8). In terms of water use and areal extent, the most important
aquifer is the highly productive Mississippi River Valley alluvial
aquifer. The minor aquifers include the Arkansas River, the Ouachita-Saline
Rivers, and the Red River alluvial aquifers. The Arkansas River
alluvial aquifer is not as widespread as the other two aquifers,
but locally is an important water source.

Parts of four Coastal Plain aquifer systems, the coastal lowlands
(fig. 9), the Mississippi embayment
(fig. 10), the Southeastern Coastal
Plain (fig. 11), and the Edwards-Trinity,
which are within rocks of Cretaceous to Quaternary age, are in
the Segment 5 area. Coastal Plain rocks of Cretaceous age make
up a locally important aquifer known as the Tokio-Woodbine aquifer.
For purposes of this chapter, the Southeastern Coastal Plain and
the Edwards-Trinity aquifer systems, and the Tokio-Woodbine aquifer
are described in the section entitled, "Cretaceous Aquifers";
only parts of the aquifer systems are present (fig.
11). Aquifers and confining units within each of the four
Coastal Plain aquifer systems thin landward to a featheredge and
thicken with

depth as they extend toward the Gulf of Mexico into the deep
subsurface. Most Segment 5 Coastal Plain aquifers contain freshwater
downgradient well beyond the extent of their outcrop. All of the
Coastal Plain aquifers and aquifer systems are comprised predominantly
of poorly consolidated to unconsolidated clastic sedimentary rocks.
The distribution and pattern of permeability within the different
Coastal Plain aquifer systems are a function of lithology and
primary porosity. In general, the most permeable Coastal Plain
aquifers consist of sand and some gravel and are separated by
silt, clay, marl, or chalk confining units. As these aquifers
extend downdip, most grade to less permeable facies, such as clay
or marl, that are part of adjoining confining units. A geopressured
zone truncates the gulfward limit of aquifers within the coastal
lowlands aquifer system.

Flat-lying to southward-dipping limestone, dolomite, and sandstone
comprise the principal aquifers of the Ozark Plateaus aquifer
system (fig. 12). Permeability within
this aquifer system is a function of regional and local tectonics,
diagenesis, geochemistry, hydrology, and weathering.

The Western Interior Plains confining system underlies the
rugged Boston Mountains and the rolling lowlands, synclinal mountains,
and cuestas that characterize the northern flank of the Arkansas
Valley. Although this confining system is poorly permeable, it
contains sandstone, minor limestone, and highly jointed and fractured
siltstone and shale that function as local aquifers. Geologic
structure is a principal factor that controls the occurrence and
movement of ground water within the unweathered part of the confining
system. Another permeability control is associated with local
faults, joints, and fractures. Ground-water movement within this
system of secondary permeability depends on the intensity, aperture,
orientation, connectivity, and filling of fracture systems.

Limited quantities of ground water can be obtained from sandstone,
siltstone, shale, and chert-novaculite rocks of the Ouachita Mountains
Section and the southern part of the Arkansas Valley Section.
Primary porosity within the Paleozoic rocks of Segment 5 was destroyed
by compaction during burial and structural deformation during
uplift. Small amounts of ground water can be obtained from wells
completed in rocks that contain joints and fractures or bedding
planes.

GEOLOGY

Segment 5 is underlain by sedimentary rocks that range from
unconsolidated to poorly consolidated clastic rocks in the Coastal
Plain Province and alluvial areas to well-consolidated, flat-lying
to southward-dipping fractured carbonate and clastic rocks in
the Ozark Province to fractured, faulted, and folded shale, sandstone,
limestone and chert-novaculite rocks in the Ouachita Province.
Coastal Plain Province rocks are Mesozoic and Cenozoic (Jurassic
to Quaternary) in age; rocks that underlie the Ozark and Ouachita
Provinces are Paleozoic (Cambrian to Pennsylvanian) in age (figs. 13, 14).

The geologic and hydrogeologic nomenclature used in this report
differs from State to State because of independent geologic interpretations
and varied distribution and lithology of rock units. A fairly
consistent nomenclature, however, can be derived from the most
commonly used rock names. Therefore, the nomenclature used in
this report is basically a synthesis of that used by the U.S.
Geological Survey, the Arkansas Geological Commission, the Louisiana
Geological Survey, and the Mississippi State Geological Survey.
Individual sources for nomenclature are listed with each correlation
chart prepared for this report.

Rocks of Ordovician to Pennsylvanian age underlie the outcrop
areas of the Ozark Plateaus Province, and are, in turn, underlain
by dolomite and sandstone beds of Cambrian age. The rocks of Cambrian
age form the basal part of the Paleozoic sedimentary sequence,
but are not exposed in northern

Arkansas. The Ozark Uplift is a structural high area that affects
the attitude of Paleozoic rocks in northern Arkansas. In general,
the rocks in northern Arkansas crop out as annular bands around
the center of the Ozark Uplift, which is located in southern Missouri
(fig. 15). Rocks of Ordovician to Mississippian
age in the Ozark Plateaus Province that dip gently southward from
northern Arkansas are dominated by shallow-water carbonate-shale
sequences and contain some prograding deltaic sandstones, all
of which were deposited on a cratonic shelf of Precambrian age.
Sedimentary rocks that underlie the Boston Mountains Section consist
mostly of Pennsylvanian sandstone and shale deposited in deltaic,
open marine, coastal, and swamp environments.

Rocks that underlie the Ouachita Province consist mostly of
a thick sequence of shale and sandstone that was deposited during
Cambrian to early Pennsylvanian time within an elongate, subsiding
Ouachita Trough. This trough formed by rifting along a late Precambrian-early
Paleozoic continental margin. Down-to-the-south normal faulting
and subsidence during Mississippian to Pennsylvanian time formed
the shallower Arkoma Basin that lies north of the Ouachita Trough.
Clastic, deep-water sediments were deposited within the Ouachita
Trough and prograding deltaic, clastic shallow-marine, and some
deep-marine deposits infilled the Arkoma Basin. Compressional
tectonic forces closed the trough during late Pennsylvanian time
and helped form the Ouachita Anticlinorium. The Ouachita Anticlinorium
is an intensely fold-ed structure of plunging synclines, anticlines,
and north- and

south-directed thrust faults; shale, chert, sandstone, conglomerate,
novaculite, and volcanic tuff of this folded structure have been
subject to widespread low-grade and low-temperature metamorphism.
To the north, rocks of Pennsylvanian age within the Arkoma Basin
were gently folded into an alternating series of synclines and
anticlines in which anticlinal axes are separated by a distance
from 5 to 8 miles. Normal faults are common in areas north of
the Arkansas River, and thrust faults are present south of the
river.

Postorogenic rocks of Permian and Triassic age rest un-conformably
on the eroded Ouachita folded rocks but lie buried beneath Coastal
Plain deposits in northern Louisiana. These strata largely consist
of continental red-bed sedimentary deposits derived from the erosion
of the Ouachita Mountains.

The oldest Coastal Plain rocks of Segment 5 are Jurassic in
age and are deeply buried in the subsurface. A thick and extensive
salt layer of Jurassic age composes the lower part of the Coastal
Plain sequence in the Gulf Coast Basin (Stage I, fig.
16). Collapse and the gulfward diapiric flow of salt occurred
during crustal downwarping and infilling of the Gulf Coast Basin
(Stage II, fig. 16). Coastal Plain
sedimentary rocks of Cretaceous age were deposited across a broad
shelf. Major Cretaceous depositional facies shifted landward or
gulfward during transgressive or regressive cycles that were controlled
by eustatic sea-level change and differential rates of subsidence.
Rocks of early Tertiary age and younger were deposited during
progradational depositional cycles of alluvial and deltaic infilling
within the Gulf Coast Basin (Stages III, IV, fig.
16).

Salt-dome basins are in southern Mississippi and central and
southern Louisiana (fig. 15). Although
few occur near the land surface, salt domes penetrate most or
all of the Tertiary rocks at isolated locations. The domes are
usually only 1 to 3 miles in diameter, and their effect on ground-water
flow and water quality is localized.

The Mississippi Embayment is a large reentrant that forms a
southward-plunging syncline, which greatly influences the outcrop
pattern of Coastal Plain rocks in Segment 5. The embayment axis
is closely aligned with the present-day (1997) location of the
Mississippi River (fig. 15). Except
where they are covered by Holocene alluvial deposits of the ancestral
Mississippi River, Coastal Plain sedimentary rocks of Cretaceous
to early Tertiary age crop out mostly in offlapping bands that
parallel the perimeter of the embayment and dip gently toward
its axis. Younger outcropping Coastal Plain sediments of late
Eocene to Pliocene age do not extend as far north into the embayment
as do older strata, but crop out as a belt that parallels the
coastline, dipping gently southward into the Gulf Coast geosynclinal
basin. Late Quaternary alluvial and deltaic deposits of the Mississippi
River and its tributaries form a wide band that extends southward
from the northern part of the embayment into the Gulf of Mexico.
From a landward, outcropping featheredge, the entire Coastal Plain
sequence thickens greatly toward the axis of the Mississippi Embayment
and the Gulf Coast Geosyncline. The general gulfward thickening
is interrupted by uplifts, domes, anticlines, basins, synclines,
and faults of subregional size, some of which are shown in figure 15.

FRESH GROUND-WATER WITHDRAWALS

Ground water serves as an important source of water for many
of the 9 million people that live in Arkansas, Louisiana, and
Mississippi and supplies 38 percent of all water needed for public
supply, agriculture, industry, mining, thermoelectric power, domestic,
and commercial uses. About 6,800 million gallons per day of ground
water were withdrawn in the three States of Segment 5 during 1985;
about 80 percent of the ground water withdrawn is utilized by
irrigated agriculture, mostly for growing rice, commercial vegetables,
corn, and soybeans (fig. 17). The greatest
increase in withdrawal rates occurred between 1970 and 1980, mostly
within the Mississippi Alluvial Plain Section. In recent years,
irrigation pumping has caused Arkansas to withdraw larger amounts
of ground water than the other States of Segment 5 (fig.
18). Public supply represents the second largest user of ground
water but accounts for less than 10 percent of the total water
withdrawn. Ground water provides about 64 percent of the total
freshwater withdrawn in Arkansas, about 68 percent of the freshwater
used in Mississippi, and about 14 percent of the total freshwater
withdrawn in Louisiana.

Total ground-water withdrawals, by county, during 1985 are
shown in fig. 19. The largest withdrawals
are in two major areas-the Mississippi Alluvial Plain in eastern
Arkansas and northeastern Mississippi and a five-county area in
southwestern Louisiana. Much of the ground water withdrawn is
used for agricultural purposes, mostly irrigation of rice with
smaller amounts for aquaculture (primarily catfish farming), both
of which provide an important economic base for the three-State
area.

Total ground-water withdrawals during 1985 from each of the
major aquifers and aquifer systems are shown in fig.
20. The Mississippi River Valley alluvial aquifer and minor
alluvial aquifers (the surficial aquifer system), which are the
most heavily used, provide about 5.1 billion gallons per day of
ground water or about 75 percent of all ground water used in Segment
5. Of this amount, about 5,050 million gallons per day was withdrawn
from the Mississippi River Valley alluvial aquifer. The second
most heavily used source of ground water is the coastal lowlands
aquifer system, which provided nearly 1,150 million gallons per
day, or 17 percent of all ground water used. The Mississippi embayment
aquifer system provided about 6 percent of the total ground water
withdrawn in the three-State area, whereas Cretaceous and Paleozoic
aquifers provided only about 1 percent each of the total ground-water
withdrawals.

Withdrawals of large quantities of water from Coastal Plain
aquifer systems during the last 90 years have lowered water levels,
decreased the saturated thickness of several aquifers, caused
encroachment of salt water, and even altered patterns of regional
ground-water flow. Before development of the Coastal Plain aquifers,
recharge entered the regional flow system in the upland, interstream
areas between major rivers (fig. 21).
Ground water was discharged in the valleys of the major rivers
or along the coast. Recent regional investigations have shown
that large, long-term withdrawals have caused an increase in the
rate of recharge in some upland areas and that most of the major
rivers no longer represent sites of regional ground-water discharge
(fig. 22). Rather, because of extensive
irrigation or the lowering of ground-water levels owing to pumpage
near the rivers, most of the major river valleys have become recharge
areas that provide water to the underlying Coastal Plain aquifers.